1. Field of the Invention
The invention relates to digital television systems, and more particularly, to a method and apparatus for detecting and rejecting NTSC interference from an ATSC signal.
2. Description of the Prior Art
Please refer to FIG. 1, which is a diagram of a typical transmission frequency band conforming to the Advanced Television Systems Committee (ATSC) DTV standard and the National Television System Committee (NTSC) analog TV standard. It is well known that the NTSC analog TV utilizes three sub-carriers to transfer information. As shown in FIG. 1, these NTSC sub-carriers are utilized for transferring NTSC video information, NTSC color information, and NTSC audio information, respectively. Please note that the frequency intervals among the three sub-carriers are fixed. As shown in FIG. 1, the frequency interval between the NTSC video sub-carrier and the NTSC color sub-carrier is 3.58 MHz, and the frequency interval between the NTSC color sub-carrier and the NTSC audio sub-carrier is 0.92 MHz. Furthermore, it can be seen in FIG. 1 that the three NTSC sub-carriers overlap the transmission band defined for the ATSC signal. When an ATSC TV receives the ATSC signal, the information carried by the three NTSC sub-carriers can be regarded as the co-channel interferences of the ATSC signal. Therefore, the information carried by the three NTSC sub-carriers should be precisely removed before the ATSC TV demodulates the received ATSC signal; otherwise, the final display quality of the ATSC TV might be degraded due to such undesired interferences.
A first method in the conventional art to reject NTSC co-channel interference from the ATSC signal is to utilize a plurality of notch filters to filter out the interference components within a frequency band. If the NTSC interference sub-carrier can be placed at the null of the notch filter, the NTSC interference can be appropriately removed. However, when, as in most cases, the NTSC signal and the ATSC signal come from different transmitter sources, and thus have carrier offsets between transferred signals, it becomes practically difficult to align the nulls of the notch filters with the interference sub-carriers. The consequence is that the NTSC interferences cannot be precisely placed at the nulls, and the notch filters may filter out wrong frequency components instead of the unwanted NTSC interferences.
A second method is to utilize a comb filter to filter out the interference components within an incoming signal. The comb filter has a plurality of nulls, but usually wider in frequency. This means that even when the NTSC signal and the ATSC signal have slight carrier offsets, the comb filter can still filter out the deviated interference components. An alternative problem occurs, however. Due to the comb filter having a wider null, the comb filter simultaneously filters out the interference components and some signal components belonging to the wanted ATSC signal, whereby increases the difficulty of performing back-end decoding operation on the unduly filtered ATSC signal.